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FATTY SUBSTANCES  (GENERAL METHODS)            383

soluble fatty acids.    Thus the following oils :  Cretan Elliotianus, curcas, grape-
seed, Macassar, palm-kernel, cacao, coconut, dogfish, dolphin (from  the head)
have values varying from 87 to 94 ;   dolphin oil (from the jaw), spermaceti
and wool fat, from 59 to 66.    The number for butter is 86-90, and those for
the waxes are also comparatively low.

15. Hydroxy-acids

The determination of the quantity of hydroxy-acids contained in a
fatty substance is effected by Fahrion's method, based on the insolubility
of the hydroxy-acids and the solubility of all the fatty acids, in light petro-
leum.

From 3 to 5 grams of the fatty substance are saponified in the usual
way (see 5 : Saponification), the alcohol evaporated, the soap dissolved in
50-70 c.c. of hot water, decomposed in a separating funnel with dilute
hydrochloric acid, shaken well with 100 c.c. of petroleum ether (boiling
below 80) and left until the two separate layers are perfectly clear. The
aqueous layer is run off and then the petroleum ether, the insoluble hydroxy-
acids, which remain adherent to the walls of the funnel, being washed several
times with petroleum ether and afterwards dissolved in boiling alcohol.
The alcoholic solution is evaporated to dryness in a tared dish and the
residue dried at 100 and weighed.

This method allows of the determination of the hydroxy-acids produced
by the oxidation, either natural or artificial, of an oil or fat. Such a determina-
tion has special importance in the analysis of boiled linseed oil and of the so-
called blown oils, which are rich in hydroxy-acids.

16. Lactones or Internal Anhydrides

The simplest method of determining the content in internal anhydrides
of a mixture of fatty acids is based on the following principle : in a mixture
of pure insoluble fatty acids it is found that the acid number is equal to
the saponification number, so that there is no ester number. If, however,
the fatty acids are accompanied by lactonic anhydrides, the saponification
number differs from the acid number. This is because the fatty acids are
saturated immediately in the cold by potash, whilst the lactones must be
boiled with excess of alcoholic potash in order to be neutralised.

Hence, to ascertain the content in lactones of a mixture of fatty acids,
it is sufficient to determine by the ordinary methods the acid number and
the saponification number and, consequently, the ester number. From
the latter the content of lactone may be calculated, when the molecular
weight from which the ester number is calculated is known (usually the
lactone content is calculated as stearolactone).

In order that the acid, saponification and ester numbers of the fatty
acids may not be confused with the respective numbers for the fatty sub-
stances, it has been proposed to call the former: Constant acid number,
constant saponification number and constant ester number.

EXAMPLE : If a mixture of fatty acids gives the constant acid number
160 and the constant saponification number 195, the constant ester number
will be 35,ummy or gelatinous substances, etc.